(1) Field of the Invention
The invention relates to a liquid crystal panel, a display and a driving method thereof, and more particularly to a method for uniform brightness in a liquid crystal panel or display with wide viewing angle characteristics.
(2) Description of the Related Art
Based on smaller, lighter, lower driving voltage and wider application, liquid crystal display (LCD) replaces the cathode ray tube (CRT) gradually. However, the nature of the liquid crystal material restricts the development of the LCD with wide viewing angle characteristics. Due to the shortcoming of the narrow viewing angle, observers are easy to feel noticeable color shift in the LCD.
Color shift is a phenomena that color varies with the viewing angle of the LCD. One cause of color shift is that liquid crystal molecules exhibit different light transmission in different viewing directions for observers. It causes brightness variation in different viewing direction, so that the white balance of the LCD is destroyed. For instance, a white image may become bluer or more yellow. Aqua, lawn-green or a color of skin are brighter with wider viewing angle. If these colors are further mixed with other colors, they becomes whiter than which color they should be. Generally, color shift occurs in most of the LCDs, particularly in VA mode LCDs.
FIG. 1 shows the way how to define the viewing angle. Point A is a base point on a display surface 10. There are innumerable planes, relative to the display surface 10, including a transverse plane defined as angle φ equal to 0 degree and a longitudinal plane defined as angle φ equal to 90 degree. The angle θ is between a normal line 11 of the display surface 10 and a line of sight from the observer. For example, angle θ equal to 0 degree represents that the observer is located over the display surface 10. In the transverse plane (angle φ equal to 0 degree), angle θ equal to 90 degree represents that the observer is located at the left/right of the display surface 10. In the longitudinal plane (angle φ equal to 90 degree), angle θ equal to 90 degree represents that the observer is located in front/rear of the display surface 10. For simplification, we define angle θ as a viewing angle in this article. A specific angle is called the largest viewing angle, at which the observer can not figure out the color shown on LCDs.
Referring to FIG. 2A and FIG. 2B, a pixel array and an unit pixel of the conventional LCD are shown. A liquid crystal panel 20 comprises an upper substrate 21, a lower substrate 22 and a pixel array including a plurality of unit pixels 23. For controlling brightness of the liquid crystal panel 20, each unit pixel 23 includes a common electrode 231 on the upper substrate 21, a thin film transistor (TFT) 232, a data line 233 and a scan line 234 on the lower substrate 22. The common electrode 231 is applied with a stable voltage. The TFT 232 has a source electrode 2321, a gate electrode 2322 and a drain electrode 2323. The source electrode 2321 is connected to the data line 233. The gate electrode 2322 is connected to the scan line 234. The drain electrode 2323 is connected to a pixel electrode 235. The ON/OFF control of the scan line 234 is performed by a two-level driving method.
Still referring to FIG. 2A-2B, after an external voltage inputs to the source electrode 2321 through data line 233 and drops, a drain voltage is created and passes to the pixel electrode 235. The absolute value of the voltage difference between the pixel electrode 235 and the common electrode 231 is called a driving voltage. The driving voltage affects the arrangement of the liquid crystal molecules 241 in the liquid crystal layer 24. Hence, the tilt direction of the liquid crystal molecules 241 varies with the external voltage, so as to change the transmission and the brightness of the liquid crystal panel 20. In order to maintain the driving voltage until the next external voltage inputs, a storage capacitor 236 is applied to the unit pixel 23. The storage capacitor(Cs) 236 has one end connected to the pixel electrode 235, and the other end connected to the scan line 234 of the former unit pixel (Cs on gate structure) or to the common electrode 231 (Cs on common structure).
FIG. 3A is a chart showing transmission-external voltage characteristic of a VA-mode liquid crystal display. The abscissa is the external voltage (V), and the ordinate is the transmission with a dimensionless unit (%). Curve 31 and curve 32 respectively represents the transmission-external voltage characteristic at viewing angle θ equal to 0 degree (front view) and 60 degree. The two curves 31 and 32 show that the liquid crystal molecules exhibit different transmission at different viewing angle θ when the same external voltage input to the unit pixel. And when the external voltage exceeds a threshold voltage, the liquid crystal molecules start to rotate to change the transmission. In FIG. 3A, the threshold voltage is about 2V.
FIG. 3B shows gamma curves of the VA-mode liquid crystal display. The abscissa is marked with 0-255 gray levels. The ordinate represents the transmission which is a normalized value with a dimensionless unit, and its maximum value is defined as 1. For each below 210 gray level, the transmission increases as the viewing angle increases. Under these gray levels, the larger viewing angle corresponds to more brightness.
To sum up, conventional method for controlling brightness can not overcome non-uniform brightness and color shift. The reason is that when supplying one external voltage to one unit pixel of the convention LCD, all liquid crystal molecules of the unit pixel will be driven to orient to the same direction. Therefore, the liquid crystal molecules guide the light to emit in the same direction. FIG. 4 is the orientation of the liquid crystal molecules in the unit pixel 40 having two different transmissions. The dashed line represents low transmission. Various techniques for overcoming non-uniform brightness or color shift are changing alignment films or adding a wide viewing film. These methods need to change a manufacturing process, so that they cause a brightness lost and a by-effect such as image sticking and cross talk etc.